U.S. Pat. No. 5,610,619 issued to Safari Mar. 11, 1997, entitled "Backlite Antenna for AM/FM Automobile Radio Having Broadband FM Reception," discloses a backlite antenna system including a separate defogger grid and an AM/FM antenna grid. The antenna grid includes antenna elements that extend across the rear window of the vehicle that are connected by antenna end bars. A tuning stub that provides FM reception is connected to the antenna grid and is positioned between the antenna grid and the defogger grid. The defogger grid includes defogger elements that extend across the window and are connected by defogger end bars. The defogger grid is heated by applying a current to one of the end bars and grounding the other end bar.
The antenna elements and the defogger elements are not directly connected to each other, but are spaced apart so that the defogger elements are electromagnetically coupled to the antenna elements and are driven as a parasitic element. Since the defogger elements cover most of the viewing area of the rear window, the antenna elements are confined to an upper portion of the window. The vehicle body acts as a ground plane and is capacitively coupled to the antenna elements through a urethane seal that seals the rear window to a vehicle body flange. It is important to control the smallest distance between the antenna elements and the body metal ground plane to control the antenna impedance.
In this type of design, the defogger grid acts as a parasitic antenna element. The parasitic,element affects the antenna's vertically polarized FM reception characteristics. Improvements in the FM vertically polarized antenna characteristics are achieved by providing two vertical shorting bars near the center of the defogger grid. The shorting bars ground the center portion of the defogger grid to provide a consistent ground plane across the entire width of the defogger grid.
The defogger grid also causes AM noise to be received by the antenna elements. The AM noise results from the defogger grid current and vehicle generated noise. This noise is reduced by using the existing DC ground for the defogger grid current connected to one side of the defogger grid. In one design, this is a long wire connected to one end bar of the defogger grid and vehicle ground. Additionally, an RF ground circuit is connected to the other end bar of the defogger grid, and includes a capacitor connected to this end bar and vehicle ground. This additional RF grid ground provides a low impedance path at AM and FM frequencies and a high impedance path at DC. The RF ground circuit needs to provide a very high DC impedance path so that it will not shunt to ground the defogger grid current and defeat the purpose of the defogger grid.
Although the separated AM/FM antenna grid and defogger grid have been successful in providing AM/FM reception and performance, improvements can be made to reduce costs, improve noise reduction, and provide improved FM impedance matching. For example, the costs involved in providing and installing the RF ground circuit can be reduced or eliminated. It is an object of the present invention to provide a vehicle backlite antenna system including an improved RF ground circuit.